That would be front seat car sex with my wife, many years ago now. As we we driving home from eating out I mentioned how I wanted her for dessert when we got home, and some sext talk led to my huge erection. She felt me up, then unzipped me stroked me then actually went down on me (she is very very good at that) while I was driving on the highway. She stopped before I exploded as that would have been dangerous, but as soon as we parked the car at home she removed her panties and climbed over and lowered herself on my awaiting erection. That was a little bit uncomfortable (mostly for her) but she had to have me right then and there.
Such an interesting question, so I will repeat it:
When was the last time (if ever) that you had sex in a non-traditional space (i.e. not in a bedroom or your personal living space)?